In the new Office 2013 and SharePoint 2013 worlds, add-ins are passe. Apps are in.

This is the crux of what Microsoft officials mean when they talk about the new "cloud app model" that the Office team has adopted with its coming products.

This new model -- complete with new Office and SharePoint Stores for marketing/selling/deploying apps from Microsoft and third-party developers -- is "the most significant thing we've done with the (Office) platform in 15 years" since the introduction of Visual Basic for Applications (VBA), said Brian Jones, Group Program Manager on the Office Solutions Framework team.

First things first: Developers who want to build these new apps -- and take home 80 percent of the revenues for paid ones (with Microsoft keeping the other 20 percent) -- need to think different. No more using VBA or VSTO or macros if you want your app to be available via the new Office and SharePoint Stores. While VBA, VSTO, macros and other legacy Office development concepts and conventions will still work with the coming versions of Office and SharePoint, Jones said, the way of the future for Office developers is via the Web.

In the Apps for Office and Apps for SharePoint context, "via the Web" means, very simply, that apps be able to talk OAuth, REST and other Web protocols. All that gets "embedded" in these new apps is a pointer to wherever an app is hosted. That doesn't mean the app must be hosted in Windows Azure, though, of course, it can be. It also could be hosted on a vendor's on-premises server, on Amazon, CloudStack, or any other public or private cloud site.

As a result of this pointer/Web structure, apps can follow users when they are signed into Office. The apps a user downloads "go with" the documents with which they are associated. (If a user doesn't have a particular app installed that is required for viewing/modifying an app, there will be some kind of cue or deprecated view -- depending on what the app developer decides -- to allow such interactions.) This new model also should improve the deployability, updatability and manageability of the new generation of apps, as compared with their plug-in counterparts.

For those who appreciate architectural diagrams, here's Microsoft's picture of what the internals of an App for Office looks like:

There are two types of Apps for Office offerings: The task pane app and the content app. (These were codenamed "Agaves" not so long ago.) Content apps let users extend apps with custom content. Task pane apps also allow custom-content extensions, but this content shows up in a task pane, side-by-side with an Office document.

There are also Mail apps, Access apps and Project apps which are part of this new model. (Just to try to bring a little clarity to what I and some others have found to be a confusing naming convention, none of these new types of apps is the same as "Office Web Apps," which are the Webified versions of Word, Excel, PowerPoint and OneNote. And that's it.) In the future, templates -- thousands of which are available today via Office.com -- also could potentially be remade as Apps for Office or Apps for SharePoint.

Apps aren't the only way to extend Office 2013, SharePoint 2013, Outlook 2013, Access 2013 and Project 2013. Users also are going to be able to combine these new apps with datasets from Microsoft and third parties. There's a freely downloadable app that pulls in real-time data about Olympic gold medals earned by each country known as "Medal Tracker" that shows off this capability. Microsoft is exploring ways to tie its Windows Azure Marketplace data feeds into these new apps, as well, officials said.

On the SharePoint side of the house, there's a similar story. "Everything is now considered an app," quipped Richard Riley, Director of Product Management for SharePoint. It's not just third-party content that gets designated as an app. Tasks in SharePoint also become apps; calendars become apps. And all of these apps are available via the SharePoint store, which is built directly into SharePoint 2013, and shares the same back-end as the new Office Store, which is hosted on Microsoft's Office.com site.

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